Nitride semiconductor device

ABSTRACT

There is provided a nitride semiconductor device including: an n-type nitride semiconductor layer; a p-type nitride semiconductor layer; and an active layer formed between the n-type and p-type nitride semiconductor layers, the active layer including a plurality of quantum well layers and at least one quantum barrier layer deposited alternately with each other, wherein the active layer includes a first quantum well layer, a second quantum well layer formed adjacent to the first quantum well layer toward the p-type nitride semiconductor layer and having a quantum level higher than a quantum level of the first quantum well layer, and a tunneling quantum barrier layer formed between the first and second quantum well layers and having a thickness enabling a carrier to be tunneled therethrough.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No.2007-0126131 filed on Dec. 6, 2007, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor device, andmore particularly, to a nitride semiconductor device improved inemission efficiency due to an active layer having an optimal structureof quantum barrier and quantum well layers, notably, when operating in ahigh current.

2. Description of the Related Art

In general, a nitride semiconductor is broadly utilized in a green orblue light emitting diode (LED) or a laser diode (LD) which serves as alight source in a full color display, an image scanner, various signalsystems and optical communication devices. This nitride semiconductordevice may act as a light emitting device including an active layeremitting light of various colors such as green and yellow by virtue ofrecombination of electrons and holes.

Since development of the nitride LED, technological advancement has beenmade remarkably to broaden the scope of application of the nitride LED.Accordingly, the LED has been significantly researched as a light sourcefor general lighting. Particularly, conventionally the nitride lightemitting device has been mainly used as parts employed in a mobileproduct of low current and low output. However, recently, the nitridelight emitting device has seen its application expanding to the highcurrent and high output field. This has led to an urgent need fordeveloping an LED structure with high efficiency in a high current.

FIG. 1 is a cross-sectional view illustrating a conventional nitridesemiconductor device.

Referring to FIG. 1, the nitride semiconductor device 10 includes asapphire substrate 11, and an n-type nitride semiconductor layer 12, anactive layer 15 of a multiple quantum well structure, a p-type nitridesemiconductor layer 17 and a transparent electrode layer 18 formedsequentially on the sapphire substrate 11.

The n-type nitride semiconductor layer 12 is partially etched to providean area for forming an n-electrode 19 a. A p-electrode 19 b is formed onthe transparent electrode layer 18.

Here, the active layer 15 is formed of a multiple quantum well structurehaving a plurality of quantum well layers 15 a and quantum barrierlayers 15 b deposited alternately with each other.

This nitride semiconductor device has emission efficiency determinedlargely by recombination probability of electrons and holes in theactive layer, i.e., internal quantum efficiency.

To enhance internal quantum efficiency, studies have been directed atincreasing the number of effective carriers involved in light emissionby improving a structure of the active layer. That is, to ensure agreater number of effective carriers in the active layer, there has beena need to reduce the number of carriers overflowing outside of theactive layer.

Also, carriers are limitedly injected to a specific local area of theactive layer to thereby reduce an effective active area in the totalactive layer. Such a decline in the effective active area directly leadsto degradation in light emitting efficiency. This accordingly has calledfor a method for assuring recombination in the entire active layer.

Amore detailed description will be given with reference to FIG. 2.

FIG. 2A illustrates a simulation result for carrier concentration of anactive layer having seven pairs of quantum well layers and quantumbarrier layers with a thickness of 30□, and 150□, respectively in aconventional nitride semiconductor device. FIG. 2B illustrates asimulation result for rediative recombination rate of an active layerhaving seven pairs of quantum well layers and quantum barrier layerswith a thickness of 30□, and 150□, respectively in a conventionalnitride semiconductor device.

First, according to the carrier concentration (electrons indicated by adotted line and holes indicated by a solid line) shown in FIG. 2A, theholes are relatively less mobile than the electrons and thus withincrease in the number of pairs, the holes are far less likely tosurvive. With a greater distance of the electrons and holes from then-type and p-type nitride semiconductor layers, respectively, theelectrons and holes are less distributed. But the holes are relativelymore rapidly decreased. Thus, as shown in FIG. 2B, effectiverecombination probability is shown high in a quantum well layer locatedin an area II near the p-type nitride semiconductor layer.

The effective recombination probability of the active layer as describedabove may be much further decreased notably when utilized in a lightingdevice requiring a high current. Therefore, this has led to a need inthe art for a multiple quantum well structure capable of increasingemission efficiency when the light emitting device operates in a highcurrent.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a nitride semiconductordevice significantly improved in emission efficiency when operating in ahigh currency due to an optimal multiple quantum well structure.

According to an aspect of the present invention, there is provided anitride semiconductor device including: an n-type nitride semiconductorlayer; a p-type nitride semiconductor layer; and an active layer formedbetween the n-type and p-type nitride semiconductor layers, the activelayer including a plurality of quantum well layers and at least onequantum barrier layer deposited alternately with each other, wherein theactive layer includes a first quantum well layer, a second quantum welllayer formed adjacent to the first quantum well layer toward the p-typenitride semiconductor layer and having a quantum level higher than aquantum level of the first quantum well layer, and a tunneling quantumbarrier layer formed between the first and second quantum well layersand having a thickness enabling a carrier to be tunneled therethrough.

The active layer may include a plurality of quantum barrier layers, andone of the quantum barrier layers is formed adjacent to the secondquantum well layer toward the p-type nitride semiconductor layer,wherein the one quantum barrier layer is a crystal quality improvementlayer having a thickness greater than a thickness of the tunnelingquantum barrier layer.

The active layer may include the first quantum well layer, the tunnelingquantum barrier layer, the second quantum well layer and the crystalquality improvement layer as one unit structure, and has the unitstructure repeated at least once.

The unit structure may be repeated one to thirty times.

The second quantum well layer may have a thickness smaller than athickness of the first quantum well layer.

The first quantum well layer may have a thickness of 20 to 60□.

The second quantum well layer may have a thickness of 10 to 50□.

The tunneling quantum barrier layer may have a thickness of 10 to 80□.

The crystal quality improvement layer may have a thickness of 30 to200□.

The second quantum well layer may have the quantum level defined bydoping.

The active layer may further include: a third quantum well layer formedadjacent to the first quantum well layer toward the n-type nitridesemiconductor layer and having a quantum level higher than a quantumlevel of the first quantum well layer; and a second tunneling quantumbarrier layer formed between the first and third quantum well layers andhaving a thickness enabling a carrier to be tunneled therethrough.

The third quantum well layer may have a thickness of 10 to 50□.

The second tunneling quantum barrier layer has a thickness of 10 to 80□.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view illustrating a conventional nitridesemiconductor device;

FIGS. 2A and 2B are simulation results illustrating carrierconcentration and radiative recombination rate distribution of an activelayer having seven pairs of quantum well layers each having a thicknessof 30□ and quantum barrier layers each having a thickness of 150□ in aconventional nitride semiconductor device, respectively;

FIG. 3 is a cross-sectional view illustrating a nitride semiconductordevice according to an exemplary embodiment of the invention;

FIG. 4 is a magnified view illustrating an area indicated with A in FIG.3;

FIG. 5 illustrates a conduction band energy level of a multiple quantumwell structure shown in FIG. 4;

FIG. 6 illustrates a conduction band energy level of a multiple quantumwell structure employed in a nitride semiconductor device according toanother exemplary embodiment of the invention;

FIGS. 7A and 7B are graphs illustrating comparison results for emissionintensity between a conventional multiple quantum well structure and amultiple quantum well structure according to an exemplary embodiment ofthe invention; and

FIG. 8 is a graph illustrating a change in emission wavelength withrespect to a thickness of a crystal quality improvement layer in amultiple quantum well structure according to an exemplary embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the present invention will now be described indetail with reference to the accompanying drawings. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the shapes and dimensions may beexaggerated for clarity, and the same reference signs are used todesignate the same or similar components throughout.

FIG. 3 is a cross-sectional view illustrating a nitride semiconductordevice according to an exemplary embodiment of the invention. FIG. 4 isa magnified view illustrating an area indicated with A in FIG. 3.

First, referring to FIG. 3, the nitride semiconductor device 30 includesa substrate 31, an n-type nitride semiconductor layer 32, an activelayer 300 and a p-type nitride semiconductor layer 37.

The n-type nitride semiconductor layer 32 may be partially exposed tohave an n-type electrode 39 a formed on a top of the exposed portion ofthe n-type nitride semiconductor layer 32. Also, a transparent electrodelayer 38 and a p-type electrode 39 b may be sequentially formed on a topof the p-type nitride semiconductor layer 37.

The present embodiment illustrates a planar nitride semiconductor devicewhere the n-type and p-type electrodes 39 a and 39 b are arranged toface an identical direction. But the present invention is not limitedthereto but it is readily understood to those skilled in the art thatthe present invention may be applied to a vertical nitride semiconductordevice.

The substrate 31 is used for growing a nitride single crystal, and ingeneral utilizes a sapphire substrate. Also, the substrate 31 may bemade of SiC, GaN, ZnO, MgAl₂O₄, MgO, LiAlO₂ or LiGaO₂.

Although not shown, according to the present embodiment, a buffer layer,e.g., undoped GaN layer may be grown to improve crystal quality of anitride semiconductor single crystal grown on the substrate 31.

The n-type and p-type nitride semiconductor layers 32 and 37 may beformed of semiconductor materials doped with n-dopant and p-dopanthaving a composition expressed by Al_(x)In_(y)Ga_((1-x-y))N, where0≦x≦1, 0≦y≦1, and 0≦x+y≦1, respectively. Representative examples forsuch semiconductor materials include GaN, AlGaN, and InGaN. Moreover,the n-type dopant may employ Si, Ge, Se, Te or C, and the p-type dopantmay adopt Mg, Zn or Be.

The active layer 300 formed between the n-type and the p-type nitridesemiconductor layers 32 and 37 emits light having a predetermined energydue to recombination of electrons and holes. As shown in FIG. 3, theactive layer 300 is formed of a multiple quantum well structure having aplurality of quantum well layers and a plurality of quantum barrierlayers deposited alternately with each other.

Particularly, in the present embodiment, the active layer features arepeated structure of four layers including two quantum well layers andtwo quantum barrier layers deposited alternately with each other as oneunit 35. This repeated structure is designed to ensure smooth migrationof carriers in the active layer.

To describe in detail the four-layer unit structure 35 of the quantumwell layers and the quantum barrier layers, an area indicated with A inFIG. 3 is magnified in FIG. 4.

As shown in FIG. 4, the multiple quantum well structure of the presentembodiment is defined by repetition of the four-layer unit structure 35including the two quantum well layers 35 a and 35 c and the two quantumbarrier layers 35 b and 35 d.

Hereinafter, the quantum well layers 35 a and 35 c and the quantumbarrier layers 35 b and 35 d are referred to as a first quantum welllayer 35 a, a tunneling quantum barrier layer 35 b, a second quantumwell layer 35 c and a crystal quality improvement layer 35 d based onfunctional considerations.

The first quantum well layer 35 a is adjacent to the n-type nitridesemiconductor layer 32, from which electrons are injected mostprimarily. The first quantum well layer serves as a major light emittinglayer in the four-layer unit structure 35.

The tunneling quantum barrier layer 35 b has a thickness d2 enablingcarriers from the first quantum well layer 35 a or the second quantumwell layer 35 c to be tunneled therethrough. This accordingly allows thecarriers to migrate smoothly to an adjacent one of the quantum welllayers.

The second quantum well layer 35 c has a quantum level higher than aquantum level of the first quantum well layer 35 a. To this end, in thepresent embodiment, the second quantum well layer 35 c has a thicknesssmaller than a thickness of the first quantum well layer 35 a. As willbe described later, the second quantum well layer 35 c with a highquantum level mainly performs a step-like function to allow the carriersto migrate easily to the adjacent quantum well layer, and performsrelatively weak light emission function.

The crystal quality improvement layer 35 d is a quantum barrier layerfor overcoming a problem with decline in crystal quality resulting fromthe first quantum well layer 35 a, tunneling quantum barrier layer 35 band second quantum well layer 35 c deposited previously to have athickness of tens of □. That is, in the multiple quantum well structureof the present embodiment, structural features of the first quantum welllayer 35 a, tunneling quantum barrier layer 35 b and second quantum welllayer 35 c may increase mobility of the carriers in the active layer.This accordingly allows the crystal quality improvement layer 35 d,i.e., adjacent quantum barrier layer to be grown with relative greatthickness for recovering crystal quality degradation from multiple thinlayer doposition.

Therefore, the crystal quality improvement layer 35 d has a thickness d4greater than a thickness of the tunneling quantum barrier layer 35 b.However, an adequate thickness limitation of the crystal qualityimprovement layer 35 d is not essential according to the presentinvention. The crystal quality improvement layer 35 d may have athickness adequately adjusted in view of the thickness of the activelayer 300 and blue shift phenomenon, which will be described later.

Hereinafter, the function of the four-layer unit structure 35 will bedescribed in more detail with reference to FIG. 5. FIG. 5 illustrates aconduction band energy level of a multiple quantum well structure shownin FIG. 4. For explanatory convenience, FIG. 5 shows only migration ofelectrons as carriers.

First, portions of electrons e-injected into the first quantum welllayer 35 a emit light of a predetermined wavelength by recombinationwith holes for light emission. When the electrons are injected in agreat amount, the first quantum well layer 35 a has an energy level E0and E1 filled with the electrons, and extra electrons are tunneledthrough the adjacent tunneling quantum barrier layer 35 b and injectedinto the second quantum well layer 35 c.

Here, as will be described later, the second quantum well layer 35 c hasa quantum level higher than a quantum level of the first quantum welllayer 35 a. Accordingly, the electrons can be tunneled more easily froma high quantum level of the first quantum well layer 35 a to a zeroquantum level E′₀ of the second quantum well layer 35 c.

To perform this tunneling function, the tunneling quantum barrier layer35 b has a thickness d2 of about 10 to 80□. Also, the first quantum welllayer 35 a has a thickness d1 of 20 to 60□ to possess high internalquantum efficiency due to quantum effects.

The electrons injected into the second quantum well layer 35 c bytunneling have a quantum level higher than a quantum level of the firstquantum well layer 35 a. As described above, it is construed that thesecond quantum well layer 35 c mainly performs a step-like functionenabling the electrons to migrate to other adjacent quantum well layer,specifically, another first quantum well layer of the next 4-layer unit.

To achieve the high quantum level, the second quantum well layer 35 cmay have a thickness smaller than a thickness of the first quantum welllayer 35 a. Particularly, the second quantum well layer 35 c has athickness d3 of 10 to 50□.

As described above, the second quantum well layer 35 c with a highquantum level allows the electrons to be injected to the adjacentquantum well layer with greater efficiency, thereby broadening anoverall effective active area of the active layer.

Meanwhile, to achieve the high quantum level of the second quantum welllayer 35 c, the second quantum well layer may have a relatively smallerthickness. Alternatively, the second quantum well layer may be dopedwith an appropriate material or adjusted in indium or aluminum content.

The crystal quality improvement layer 35 d may have a thickness as smallas e.g., that of the first quantum well layer 35 a, tunneling quantumbarrier layer 35 b and second quantum well layer 35 c, respectively tobeneficially ensure the electrons to be injected to the adjacent quantumwell layer. However, in the present embodiment, the crystal qualityimprovement layer 35 d is formed mainly to improve crystal quality.

That is, the first quantum well layer 35 a, tunneling quantum barrierlayer 35 b and second quantum well layer 35 c having a relatively smallthickness to perform functions described above do not exhibit superiorcrystal quality. These layers with the small thickness, when depositedrepeatedly, allow the carriers to be injected with efficiency but maynot bring about big increase in overall light emission efficiency due todecline in crystal quality.

Therefore, the crystal quality improvement layer 35 d may have arelatively greater thickness than that of each of the layers 35 a, 35 b,and 35 c previously deposited.

However, as will be described with reference to FIG. 9, the crystalquality improvement layer 35 d with too great a thickness leads toincrease in the supplied current, thereby aggravating blue shift ofemitted light wavelength. Considering this, the crystal qualityimprovement layer 35 d may have a thickness d4 of 30 to 200□.

Furthermore, the multiple quantum well structure of the presentembodiment has a feature such that the first quantum well layer 35 a,tunneling quantum barrier layer 35 b, second quantum well layer 35 c andcrystal quality improvement layer 35 d constitute one unit structure 35and this unit structure 35 is repeated multiple times. This accordinglyimproves carrier mobility between the quantum well layers, andsemiconductor crystal quality. Particularly, the light emitting deviceachieves superior light emitting efficiency when operating in a highcurrent.

Here, the active layer 300 may feature repetition of only one unitstructure 35 or a plurality of unit structures. The unit structures 35may be repeated in optimal numbers according to driving current densityof a device. Generally, increase in current density leads to increase inoptimal repetition numbers. The unit structures 35, when repeatedmultiple times, may be repeated 30 times or less. That is, the number ofquantum well layers and quantum barrier layers may be sixty or less,respectively.

Meanwhile, the quantum well layers 35 a and 35 c and quantum barrierlayers 35 b and 35 d may be structured, for example, such that InGaN andGaN are repeatedly deposited. In this case, the quantum well layers andquantum barrier layers may have accurate compositions properly selectedaccording to required wavelength.

FIG. 6 illustrates a conduction band energy level of a multiple quantumwell structure employed in a nitride semiconductor device according toanother exemplary embodiment of the invention.

In the present embodiment, the unit structure of the multiple quantumwell structure includes two more layers compared with the previousembodiment.

That is, as shown in FIG. 6, the unit structure 65 of the multiplequantum well structure according to the present embodiment, in the samemanner as FIG. 5, includes a first quantum well layer 65 c, a firsttunneling quantum barrier layer 65 d, a second quantum well layer 65 eand a crystal quality improvement layer 65 f. In addition, the unitstructure 65 further includes a third quantum well layer 65 a and asecond tunneling quantum barrier layer 65 b.

In the present embodiment, not only mobility of electrons but alsomobility of holes is considered. The third quantum well layer 65 a isadditionally disposed on a migration path of the holes to allow theholes to be injected easily into an adjacent one of the quantum welllayers. The tunneling quantum barrier layer 65 b is employed for thesame purpose as the first tunneling quantum barrier layer 65 d.

That is, only difference between the present embodiment and the previousembodiment lies in carriers whose injection efficiency will be improved.The third quantum well layer 65 a and the second tunneling quantumbarrier layer 65 b are equivalent to the second quantum well layer 65 eand the first tunneling quantum barrier layer 65 d, respectively.

Therefore, FIG. 6 shows overall carrier mobility similar to that of FIG.5. However, the electrons e-injected into the third quantum well layer65 a are injected into the first quantum well layer 65 c by tunneling.Therefore, in the same manner as FIG. 5, in the present embodiment, thefirst quantum well layer 65 c also serves as a major light emittinglayer.

Carrier mobility after migration through the first quantum well layer 65c and function of each layer can be described with reference to theprevious embodiment without going into further detail.

Hereinafter, a description will be given of how much improvement isachieved by the multiple quantum well structure of the presentembodiments compared to the conventional multiple quantum wellstructure.

FIGS. 7A and 7B are graphs illustrating comparison results for emissionintensity between a conventional multiple quantum well structure and amultiple quantum well structure according to an exemplary embodiment ofthe invention.

First, FIG. 7A shows comparison results between Inventive Example andComparative Example when a chip has a size of 1 mm×1 mm and a currentdensity of 10 A/cm². Here, in Inventive Example, like a structure ofFIG. 3, four layers constitute one unit structure. Specifically, thefirst quantum well layer has a thickness of 30□, the tunneling quantumbarrier layer has a thickness of 30□, the second quantum well layer hasa thickness of 20□, and the crystal quality improvement layer has athickness of 90□. The unit structure is repeated five times.

Meanwhile, in Comparative Example, the quantum well layer and thequantum barrier layer each have a thickness of 30□ to form asuperlattice structure. A pair of the quantum well layer and a pair ofthe quantum barrier layer are repeated 14 times so that the active layerhas an overall thickness identical to Inventive Example.

As shown in FIG. 7A, the Inventive Example is increased in lightemission intensity by 22% on average compared to Comparative Example.This is because the multiple quantum well structure of a superlatticestructure is improved in carrier injection efficiency due to tunnelingbut degraded in emission efficiency due to decline in crystal quality.

In contrast, Inventive Example structured as in FIG. 3 is designed toensure easy carrier mobility and better crystal quality due to tunnelingand higher quantum level, thereby achieving superior light emittingefficiency.

FIG. 7B is a graph illustrating comparison results for light emittingintensity between Conventional Example and Inventive Examples. FIG. 7Bshows test results at a higher density of 35 A/Cm² than FIG. 8A.

Inventive Example 1 and Comparative Example of FIG. 7B are identicallystructured to the Inventive and Comparative Examples described withreference to FIG. 7A. In Inventive Example 2, as structured as in FIG.3, one unit structure includes four layers, in which a first quantumwell layer has a thickness of 30□, a tunneling quantum barrier layer hasa thickness of 30□, a second quantum well layer has a thickness of 20□,and a crystal quality improvement layer has a thickness of 50□. For allthree cases, to ensure similar thicknesses of respective active layers,the unit structure is repeated five times in Inventive Example 1, seventimes in Inventive Example 2 and fourteen times in Comparative Example.

Based on test results, Inventive Example demonstrates noticeableincrease in light emission intensity at a high current density.Particularly, higher light emitting efficiency can be achieved at a highcurrent density by increasing a repetition number of the unit structurewhile relatively thinning, not thickening, the crystal qualityimprovement layer.

FIG. 8 is a graph illustrating a change in emission wavelength withrespect to a thickness of a crystal quality improvement layer in amultiple quantum well structure according to an exemplary embodiment ofthe invention.

Referring to FIG. 8, graphs indicated with A and B show results ofComparative Example when quantum barrier layers each have a thickness of30□. Graphs C and D for Inventive Examples plot results when the crystalquality improvement layer has a thickness of 50□ and 90□, respectively.

Referring to FIG. 8, with increase in the thickness of the crystalquality improvement layer, the light emitting device suffersdeterioration in blue shift, particularly, when operating in a highcurrent.

Accordingly, the crystal quality improvement layer may have an optimalthickness determined considering test results, emission efficiencyimprovement effects of the crystal quality improvement layer and currentdensity of an actual device. In view of these factors, the crystalquality improvement layer may have a thickness of 30 to 200□.

As set forth above, a nitride semiconductor device according toexemplary embodiments of the invention is significantly improved inemission efficiency when operating in a high current due to an activelayer having an optimized multiple quantum well structure.

In addition, a semiconductor single crystal of the nitride semiconductordevice can be improved in crystal quality.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. A nitride semiconductor device comprising: an n-type nitridesemiconductor layer; a p-type nitride semiconductor layer; and an activelayer formed between the n-type and p-type nitride semiconductor layers,the active layer comprising a plurality of quantum well layers and atleast one quantum barrier layer deposited alternately with each other,wherein the active layer comprises a first quantum well layer, a secondquantum well layer formed adjacent to the first quantum well layertoward the p-type nitride semiconductor layer and having a quantum levelhigher than a quantum level of the first quantum well layer, and atunneling quantum barrier layer formed between the first and secondquantum well layers and having a thickness enabling a carrier to betunneled therethrough. 2-13. (canceled)